As a background art of this technical field, there is known an art that is described in U.S. Pat. No. 5,748,371 (Patent Literature 1), for example. This Patent Literature 1 discloses an art whereby an optical mask for modulating a phase of light is disposed in an optical system, and a point spread function (PSF, hereinafter referred to as “PSF”) is made substantially invariant within a finite distance range from an in-focus position, so that the depth of field or the depth of focus is expanded.
Although sharpness of an image picked up by the optical system in the above-mentioned Patent Literature 1 deteriorates not only in a position away from the in-focus position but also in the in-focus position, since the PSF is substantially invariant in the in-focus position and in the position away from the in-focus position, it is possible to remove deterioration of the sharpness by signal processing based on information of the above-mentioned PSF and to expand the depth of field.